Are you saying every property should have a valid_from and valid_to date or every claim? If you want that information for queries to be able to show information about deprecated properties then I don't understand the example because both queries use the same properties. If it is for claims, then I don't see what value we'd gain from storing that extra metadata. Every scenario I can think of where you care about past states of the database is already handled by the compare selected revisions feature. For example, if I use compare selected revisions on the United States of America item I can see that on March 14th Daffy Duck was the president, and now Barack Obama is. If we decide that a new name is better for a property, then we are essentially saying that it is a better name for the property when we look at past and current values of the property.

I also think that in our discussions of qualifiers for properties of datatype item we are continuing to use data that should be stored in items as examples of qualifiers. I do not expect to find information about when a president took office as a qualifier in a list of values for the heads of state property. I expect to find that information on the item for the person or on an item that contains all information about a specific presidential administration. How would we capture Grover Cleveland's terms as a qualifier? I would just expect to find a list of two dates for when he took office and two dates for when he left office on the item for Grover Cleveland. I guess we could list him twice, but then where do we put information about the vice-presidents? Do we just make an arbitrary rule that as soon as something has 3 or more qualifiers, the information should be put into separate items? I think one of the goals of Wikidata is to have each piece of information provided once, in the most appropriate place possible, otherwise maintaining consistency becomes problematic.

Also, for situations where I think there is no question that a qualifier should be used, such as historic population values, have we decided how specific of a property should be used for that qualifier? If we just have a generic date property, that allows growth to happen the fastest, because there are many places that property could be used as a qualifier. I think a date associated with a population value is pretty self-explanatory, but perhaps there are situations where people will be confused as to why there is a date associated with a value. Do we just put information in the property description or create more specific properties to use as qualifiers?

> Date: Thu, 4 Apr 2013 15:02:37 +0200
> From: benedix@zedat.fu-berlin.de
> To: wikidata-l@lists.wikimedia.org
> Subject: Re: [Wikidata-l] Page history and properties
>
> There was a discussion about this months ago and I think that the
> conclusion was every property should have a valid_from and a valid_to
> field [default = end of the universe].
>
> So you can get snapshots for past versions without any magic.
>
> An example for a hypothetic cashflow-database (dates are YYMMDD ) might be:
>
> select * from cf where origin = "wmde" and v2 = 999999 and payment_date
> >= 130404; -- would give the actual view of future cashflows coming
> from wmde
>
> select * from cf where origin = "wmde" and v2 = 121231 and payment_date
> >= 130404; -- would give the view of cashflows coming from wmde seen by
> end of year 2012
>
>
> I don't know if something like this would be easy to add to your
> existing database scheme.
>
> Lukas
>
>
> Am Do 04.04.2013 09:47, schrieb Daniel Kinzler:
> > On 03.04.2013 23:23, Bináris wrote:
> >> A good question from huwiki:
> >>
> >> When I click on an earlier version of the page in the history, will the
> >> "then-value" of the property be shown or the current value?
> >> If I read the 2013 version of [[United States of America]] in 2018, will Obama
> >> be the president?
> > You will see the current value, not the old one. This is the same as for
> > templates and images. A "time warp" system that allows us to view old versions
> > of pages exactly as they were has long been discussed, but is tricky, especially
> > when templates (or, in the case of wikidata, properties) get deleted or renamed.
> > Nobody has come up with a good solution yet.
> >
> > The qualifiers Sven mentioned will allow us to record who was president when in
> > Wikidata, but when including the "current president" on a Wikipiedia page, this
> > will always be the present one, even when looking at old version of the page.
> >
> > -- daniel
> >
>
>
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